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Sorption of CO, CH4, and N2 on Transition

Metal Ion Exchanged Zeolite-X and A

Chapter 4

Chapter-4 Sorption of CO, CH4 and N2 on Transition Metal Ion Exchanged Zeolite-X and A

116

4.1 Introduction

CO, CH4 and N2 adsorption studies in zeolites are largely confined to alkali and

alkaline earth cation exchanged zeolites. There are limited studies reported on the

adsorption of these gases in transition metal ion exchanged zeolites. This is despite

the fact that transition metal ions due to presence of valence d-shell electrons can

coordinate with adsorbate molecules and show different adsorption behaviour

compared to filled shell cations. Though, transition metal ion exchanged zeolites have

been studied for catalytic applications, their adsorption behaviour have not been

studied in depth.1-5

Scarce adsorption data on transition metal ion exchanged zeolites could be due to the

difficulty in exchanging the transition metal ions into zeolites, particularly at higher

degree of exchange, while retaining the zeolite structure. Transition metal containing

zeolites can be prepared in many ways: by ion exchange, either from aqueous

solution2-3

or by solid-state reaction4-5

, by hydrothermal synthesis6 and by adsorption

and decomposition of volatile organo-metallic compounds.7 Ion exchange from

aqueous solution has been mainly used to introduce transition-metal cations into

zeolites-A and X.1-3

If the exchanging cation is hydrolyzed, the H+ concentration in

solution may increase by several orders of magnitude, encouraging H+ exchange. The

acidic pH may also lead to zeolite framework modification, damage, destruction, or

dissolution, especially of low-silica zeolites. For example, hydrolysis of the zeolite

framework to give five –coordinated Al3+

was observed in partially Co2+

and Ni2+

exchanged zeolite-A and X.2,8-10,

For partially Co2+

exchanged zeolites, both X-ray diffraction analysis and electronic

reflectance spectroscopy have been used to determine the position of the Co2+

ions, their movement upon dehydration and the nature of their complexes. The Co2+

ions in partially dehydrated faujasite zeolites are tetrahedrally coordinated to three

framework oxygen atoms and one water molecule or its fragment (OH− or O

2−), the

Co2+

ions in fully hydrated zeolite are octahedrally coordinated to six water

molecules. Studies also showed that the Co2+

ions prefer sites I and I′ over site II in

partially dehydrated zeolite-X, whereas they move to site I upon full dehydration to

avoid three coordination and to achieve six-coordination. Far-IR spectroscopy has

been used to locate and characterize the principal transition-metal ion sites in faujasite

zeolites.10-12

Among the other transition metal elements like manganese, cadmium,


117

zinc all of which have d5 or d

10 electronic configurations, have been fully exchanged

into zeolites-X. The adsorption properties of zeolites containing more complex

transition-metal ions are established with the positions and occupancies of those ions

within the zeolite cavities. For example, silver cations in zeolites are reported to

interact strongly with the adsorbed CO and N2 molecules.13-21

These strong

interactions were attributed to electron donation by -donation from the bonding 2p

molecular orbital of N2 molecules to the empty s-orbital of the metal ions and d-2p*

back-donation from the partially or completely occupied d-orbital of metal ions to the

unoccupied 2p* antibonding molecular orbital of the N2 molecules.

The present chapter deals with the adsorption of CO, CH4 and N2 in manganese,

cobalt, nickel, copper, zinc, ruthenium, rhodium, palladium, silver and cadmium

exchanged zeolite-X and A. The adsorption data was correlated with the cation

locations and the nature of the cationic species formed inside zeolite cavities. The

CO, CH4 and N2 binary and tertiary gas mixture adsorption studies was also carried

out to find out the potential adsorbent for these gas mixture separations.

4.2. Experimental Section

4.2.1. Materials

Zeolite-A (Na96Al96Si96O384.208H2O) and zeolite-X (Na88Al88Si104O384.208H2O) in

powder as well as granule forms was procured from Zeochem LLC, Uetikon,

Switzerland, and used as received. The chloride salts of manganese, cobalt, nickel,

copper, zinc, ruthenium, rhodium, palladium, silver and cadmium (S. d. fine

Chemicals, India) are used for cation exchange. N2 (99.999%), CH4 (99.99%), CO

(99.99%) and He (99.999%) from Inox Air Products, India were used for the

adsorption isotherm measurements.

4.2.2. Transition Metal Ion Exchange

The transition metal ions were introduced into the highly crystalline Na form of the

zeolites-X and A, by the conventional cation exchange protocol from aqueous

solution.8-10,38

Detail procedure followed was same as described in Section 2.2.2. For

silver exchange all the activities are carried out in dark. The Ag+ ion exchange in

zeolite is highly facile and can occur even at ambient conditions due to high

selectivity of Ag+ over Na

+.


118

4.2.3. Characterization

X-ray powder diffraction, FT-IR, Surface Area, SEM, EDX and ICP analysis of

different transition metal ions exchanged zeolite-X and A were carried out by the

same procedure as described in Section 2.2.3.-2.2.6.

4.2.4. Equilibrium and Dynamic Adsorption Measurements

Equilibrium and Dynamic adsorption measurements of the transition metal ions

exchanged zeolite-X and A were carried out for obtaining equilibrium and dynamic

adsorption capacity and selectivity of CO, CH4, and N2 from their mixtures. The

detail procedure is same as described in Section 2.2.7 and 2.2.8.

4.3. Results and Discussion

4.3.1. X-ray Powder Diffraction

The X-ray powder diffraction patterns of the transition metal ion exchanged zeolites

(Figure 4.1a-b) indicates retention of the zeolite structure even after the transition

metal ion exchange as the major diffractions typically observed for zeolite-X at (2

theta 6.1, 10.0, 15.5, 20.1, 23.4, 26.7, 29.3, 30.5, 31.0, and 32.1) are retained.

Figure 4.1 (a-b). X-ray powder diffraction pattern of (a) Mn2+, Co2+, Ni2+, Cu2+, Zn2+ and

(b) Ru3+, Rh3+, Pd2+, Ag+ and Cd2+ zeolite-X.

The decrease in crystallinity is due to the dealumination of framework in the acidic

medium of metal salt solution during ion exchange which affect framework during

10 20 30 40 50 60

Cd(94)NaX

AgX

Pd(67)NaX

Rh(73)NaX

Ru(52)NaX

Inte

ns

ity

2 Theta

NaX

(b)

10 20 30 40 50 60

Mn(85)NaX

Co(94)NaX

NaX

Ni(72)NaX

Cu(84)NaX

Inte

ns

ity

2 Theta

Zn(95)NaX

(a)


119

vacuum activation. The percentage crystallinity for the different transition metal ion

exchanged zeolite samples are given in Table 4.1. Pd(67)NaX shows maximum

decrease in crystallinity at 623 K while Cu(84)NaX shows decrease in crystallinity at

473 K and get completely destroyed at 623 K.

4.3.2. Surface Area and Pore Volume

The unit cell composition, % crystallinity, surface area and micropore volume of

Mn2+

, Co2+

, Ni2+

, Cu2+

, Zn2+

, Ru3+

, Rh3+

, Pd2+

, Ag+ and Cd

2+ zeolite-X were

determined and given in Table 4.1. The surface area and micropore volume of zeolite-

X increases with decrease in the size and atomic weight of extra-framework cations.

The surface area and micropore volume of the zeolite-X increases on Co2+

and Ru3+

metal ion exchange. This may be due to the decrease in the number of extra

framework cations while replacing monovalent sodium ions with divalent and

trivalent ions, respectively and increase in micropore volume is due to small size

metal ion exchange.

Table 4.1. Unit cell composition, % crystallinity, micropore volume and surface area

of transition metal ion exchanged zeolite-X.

Sample Unit Cell Formula

%

Crysta-

llinity

Ionic

radii

(pm)

Micropore

Volume

(cm3/g)

BET

Surface

Area(m2/g)

Micropore

Surface

Area(m2/g)

External

Surface

Area(m2/g)

NaX Na88Al88Si104O384 100 97 0.30 692 647 45

Mn(85)NaX Mn37Na14Al88Si104O384 90 80 0.273 665 584 81

Co(94)NaX Co41Na6Al88Si104O384 72 74 0.326 802 696 106

Ni(72)NaX Ni32Na24Al88Si104O384 43 72 0.209 555 447 108

Cu(84)NaX Cu37Na14Al88Si104O384 37 69 0.089 265 175 90

Zn(95)NaX Zn42Na4Al88Si104O384 95 74 0.201 523 430 93

Ru(52)NaX Ru15Na43Al88Si104O384 58 62 0.311 727 665 62

Rh(73)NaX Rh21Na25Al88Si104O384 51 60 0.012 235 31 204

Pd(67)NaX Pd29Na30Al88Si104O384 14 86 0.260 765 672 84

AgX Ag88Al88Si104O384 98 126 0.230 548 491 57

Cd(94)NaX Cd41Na6Al88Si104O384 91 97 0.212 488 453 35

The external surface area of the transition metal ion exchanged zeolites increases,

particularly with samples exchanged with higher amount of metal ions. This could be

due to the strong interaction of bivalent and trivalent transition metal ions to zeolite


120

structure which can result in some structural defects or formation of amorphous

phase, probably due to dealumination during the cation exchange or vacuum

dehydration process. This is also evidenced by the decrease in the crystallinity of

zeolite samples exchanged with transition metal ions.

4.3.3. Activation and Colour Change

Transition metal ion exchanged zeolites are generally coloured and they show colour

change during activation due to change in electronic properties of metal ions. Mn2+

,

Zn2+

, Ru3+

, Pd2+

, and Cd2+

exchanged zeolite-X are colourless and did not show any

change in colour during activation due to half filled (d5) or full filled (d

10)

configuration whereas Co2+

, Ni2+

, Cu2+

and Rh3+

are coloured and show change in

colour during activation (Table 4.2) due to d-d transition of electrons. Ag+ (d

10)

exchanged zeolite is light gray but impart coloured during activation due to formation

of silver clusters.

Table 4.2. Change in colour of the adsorbents on vacuum dehydration.

Zeolite Colour of hydrated

sample

Colour after vacuum

activation at 623 K

NaX White White

Mn(85)NaX White White

Co(94)NaX Pink Violet

Ni(72)NaX Light green Yellow

Cu(84)NaX Blue Green

Zn(95)NaX White White

Ru(52)NaX White Gray

Rh(73)NaX Yellow Brown black

Pd(67)NaX White Black

AgX Slightly gray Golden yellow

Cd(94)NaX White White

AgA White Brick red

Silver exchanged zeolite-A reversibly change its colour from white to brick red upon

vacuum dehydration at 623 K. Ralek et al.22

first reported that the white hydrated

silver form of zeolite-A exhibits a red colour after dehydration at 623 K, which has

been later confirmed by many authors.25-37

The dehydrated silver exchanged zeolite-A


121

has been reported to adsorb visible light at 500 nm. The colour changes observed for

AgA on heating under vacuum have been attributed to the formation of (Agn)x+

clusters inside the sodalite cavity of zeolite-A. It is reported that on vacuum

dehydration silver ions migrate and undergo auto reduction to form Ag0, which

interacts with silver ions to form clusters. Various types of clusters varying from

linear Ag+-Ag

0-Ag

+ to (Ag5)

4+, (Ag8)

6+ and (Ag12)

8+ have been reported depending on

the zeolite type and the extent of silver exchange.23-34

In case of zeolite AgA, yellow

colour observed at lower temperatures (<373 K) is due to weakly interacting (Ag3)+

clusters. At higher temperatures (<600 K), the red brick colour observed is attributed

to the presence of four interacting (Ag3)+

clusters inside the sodalite cages of zeolite

AgA. However there is an alternative explanation for the formation and interaction of

(Ag3)2+

clusters often presented as responsible for the colour changes observed in

silver exchanged zeolite-A.35-37

From UV-VIS diffuse reflectance and quantum

chemical extended Huckel molecular orbital calculations on NaAgA, colour changes

are attributed to electronic transitions from the lone pairs of oxygen atoms of the

zeolite framework to the empty 5s orbital of Ag+

ions, i.e., ligand to metal transfer

(LMCT). However, in AgX, only yellow colour is observed even at higher

temperatures (723 K) under vacuum showing the presence of isolated Ag32+

clusters

in these zeolites.

4.3.4. Adsorption Isotherms and Selectivity for Mn2+

, Co2+

, Ni2+

, Cu2+

, Zn2+

,

Ru3+

, Rh3+

, Pd2+

and Cd2+

ion Exchanged Zeolite-X

The CO, CH4 and N2 adsorption isotherms at 288 and 303 K have been generated and

the equilibrium adsorption capacities for the adsorption of CO, CH4 and N2 on Mn2+

,

Co2+

, Ni2+

, Cu2+

, Zn2+

, Ru3+

, Rh3+

, Pd2+

, and Cd2+

zeolite-X were determined from the

adsorption isotherms and given in Table 4.3 and Table 4.4. All above ion exchanged

zeolites show adsorption capacity less than zeolite NaX. On replacing monovalent

sodium ions with divalent Mn2+

, Co2+

, Ni2+

, Cu2+

, Zn2+

, Pd2+

, and Cd2+

cations, one

cation replaces two Na+ ions; therefore, half of the cations are present in the zeolite

for the interaction. Similarly, on Ru3+

, Rh3+

exchange one third of cations are

available for adsorption interaction. Since cations are most important site for

adsorption of gas molecules, the decrease in their number leads to decrease in

adsorption capacity for bivalent and trivalent transition metal ion exchanged zeolite-


122

X. The decrease in crystallinity of transition metal ion exchanged zeolites also

decreases their adsorption capacity.

Table 4.3. Adsorption capacity of different transition metal ion exchanged zeolite-X

at 288 K and 760 mmHg.

Sample Adsorption Capacity at 288 K and 760 mmHg

(cm3/g) (molecules/unit cell)

CO CH4 N2 CO CH4 N2

NaX 36.90 22.79 12.83 21.01 12.97 7.31

Mn(85)NaX 20.08 15.67 13.11 11.71 9.14 7.65

Co(94)NaX 24.64 20.09 13.83 14.58 11.89 8.18

Ni(72)NaX 20.49 9.15 5.40 12.02 5.36 3.16

Cu(84)NaA 9.23 7.38 2.74 5.51 4.40 1.63

Zn(95)NaX 11.81 8.81 4.76 7.13 5.32 2.87

Ru(52)NaX 23.01 17.40 9.42 13.56 10.26 5.54

Rh(73)NaX 19.60 13.50 5.33 11.75 8.09 3.18

Pd(67)NaX 10.52 9.74 4.41 6.76 6.24 2.83

Cd(94)NaX 24.40 16.48 10.61 16.70 11.28 7.26

The major interactions of CO, CH4 and N2 with Mn2+

, Co2+

, Ni2+

, Cu2+

, Zn2+

, Ru3+

,

Rh3+

, Pd2+

, and Cd2+

zeolite-X are field gradient quadrupole, field induced dipole and

field dipole interactions which are directly proportional to charge and inversely

proportional to ionic radii of cations (Table 4.2). Transition metal ions have high

charge density so they cause high electrostatic interaction and hence have high heat of

adsorption. The high CO adsorption capacity and heat of adsorption in low pressure

region is due to formation of metal carbonyls. Due to strong interaction of the CO,

CH4, and N2 molecules they come very close to the cations and shield them strongly

to hinder their interactions with other gas molecules and hence leads to decrease in

adsorption capacity. The shielding effect is more effective for bivalent and trivalent

metal ions.


123

Table 4.4. Adsorption capacity of different transition metal ion exchanged zeolite-X

at 303 K and 760 mmHg.



CO CH4 N2 CO CH4 N2

NaX 27.17 14.00 9.2 14.71 7.57 4.98

Mn(85)NaX 9.66 12.47 10.12 5.35 6.91 5.61

Co(94)NaX 16.62 11.93 10.43 9.34 6.71 5.87

Ni(72)NaX 17.42 7.71 3.49 9.71 4.30 1.94

Cu(84)NaA 6.93 5.09 1.95 3.93 2.88 1.11

Zn(95)NaX 9.63 7.62 2.90 5.52 4.37 1.66

Ru(52)NaX 17.23 13.22 6.01 9.64 7.40 3.36

Rh(73)NaX 16.91 11.12 3.62 9.63 6.27 2.05

Pd(67)NaX 6.50 8.60 2.94 3.97 5.26 1.77

Cd(94)NaX 22.70 15.94 7.38 14.77 10.37 4.80

Table 4.5. Adsorption selectivity for different transition metal ion exchanged zeolite-

X at 288 and 303 K and 760 mmHg.

Sample Selectivity at 760 mmHg

288 K 303 K

CO/CH4 CO/N2 CH4/N2 CO/ CH4 CO/ N2 CH4/ N2

NaX 1.62 2.88 1.78 1.94 2.95 1.52

Mn(85)NaX 1.28 1.53 1.20 0.77 1.23 0.95

Co(94)NaX 1.23 1.78 1.45 1.39 1.14 1.59

Ni(72)NaX 2.24 3.8 1.70 2.26 2.21 5.04

Cu(84)NaX 1.25 3.37 2.69 1.36 2.61 3.55

Zn(95)NaX 1.34 2.48 1.85 1.26 2.63 3.32

Ru(52)NaX 1.32 2.45 1.85 1.30 2.87 2.23

Rh(73)NaX 1.45 3.70 2.54 1.54 4.69 3.12

Pd(67)NaX 1.08 2.39 2.20 0.76 2.24 3.04

Cd(94)NaX 2.06 3.32 1.62 1.42 2.16 3.08


124

The adsorption isotherm clearly shows CO selectivity over CH4 and N2 which is high

in low pressure region. The pure component adsorption selectivity for CO over CH4

and N2, and CH4 over N2 at 760 mmHg equilibrium pressures was calculated, and the

values at 288 and 303 K are given in Table 4.5.

4.3.5. Adsorption Isotherms and Selectivity for Ag+ Exchanged Zeolite-X

CO, CH4 and N2 adsorption isotherms on fully silver exchanged zeolite-X (AgX) at

288 and 303 K are given in Figure 4.2. The equilibrium adsorption capacities for the

adsorption of CO, CH4 and N2 on zeolite-X containing different amounts of silver

ions are determined from the adsorption isotherms and the values at 760 mmHg are

given in Table 4.6 and 4.7. It is observed that on silver ion exchange, CO adsorption

values show more than threefold and fourfold increase at all equilibrium pressures at

288 and 303 K respectively, as compared to NaX. However, the increase in the CH4

and N2 adsorption at 288 and 303 K is more than twofold and threefold respectively.

In the low pressure region, the CO, CH4 and N2 adsorption capacity increases sharply

with increase in pressure and the adsorption isotherm has very high slope as

compared to NaX due to strong interaction between gas molecules and silver clusters

formed during vacuum activation after more than 80% exchange. However, the

magnitude of increase in CH4 and N2 adsorption capacity is less than that of AgA as

silver clusters formed in the hexagonal prism are not accessible for adsorption while

those formed in sodalite cages are only accessible for adsorption through S6R. At

equilibrium pressures above 250 mmHg, the slope of the adsorption isotherm

decreases.

CO, CH4 and N2 adsorption isotherms on different percentage of silver exchanged

zeolite-X at 288 and 303 K are given in Figure 4.2. The number within brackets

shows the percentage of silver ion exchanged in the zeolite-X, i.e. 80% and 90%,

silver exchanged zeolite-X are shown as Ag(80)NaX and Ag(90)NaX respectively.

Ag(90)NaX shows adsorption nature same as AgX with twofold and threefold

increase in adsorption capacity at 288 and 303 K respectively. For Ag(80)NaX the

CO, CH4 and N2 adsorption capacities increase with increase in the amount of silver.

However, the increase in adsorption capacities is linear as silver ions do not interact

as strongly as silver cluster interacts, which are not formed in low silver zeolites and

hence the shape of adsorption isotherms remains linear. The observed relative

increase in the adsorption capacity follows the order CH4 ≈ N2 > CO. The Ag+


125

exchanged zeolite-X shows CO selectivity over CH4 and N2 which is very high in low

pressure region and follows the order Ag(80)NaX > Ag(90)NaX > AgX > NaA

(Table 4.8). Ag+

exchanged zeolite-X shows CH4 adsorption capacity higher than that

of N2 at all equilibrium pressures in the pressure range studied. However, with

increase in pressure there is no significant change in CH4 selectivity over N2. It is

observed that on silver ion exchange the relative increase in adsorption capacity was

more at higher temperature.

Table 4.6. Adsorption capacity for Ag+ ion exchanged zeolite-A and X at 288 K and

760 mmHg.



CO CH4 N2 CO CH4 N2

NaA 35.83 17.42 10.77 20.65 10.03 6.21

Ag(62)NaA 56.12 21.61 11.23 44.46 17.12 8.87

Ag(70)NaA 59.20 24.22 11.84 52.19 21.41 10.44

AgA 61.71 38.54 23.87 57.17 35.50 21.94

NaX 36.90 22.79 12.83 21.01 12.98 7.31

Ag(80)NaX 59.13 22.16 13.63 48.51 18.06 11.16

Ag(90)X 62.45 27.14 15.30 52.89 23.12 13.05

AgX 67.18 31.7 19.08 59.32 28.07 16.82

The increase in adsorption capacity on silver ion exchange is associated with cation

positions in dehydrated zeolite-X. The silver cations preference follows the order site

I > site I’> I’b > II’ > II > II* > III. The silver cations prefer sites at hexagonal prism

(site I, site I’ and I’b) than sodalite cage (II’, II and II*) and least preference is for

super cage (site III) where cations are easily accessible for adsorption.


126

Figure 4.2 (a-f). Adsorption isotherms of CO, CH4, and N2 in silver ion exchanged zeolite-

X (a-c) at 288 K and (d-f) at 303 K.

0 200 400 600 8000

10

20

30

40

50

60

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(a)

Ag(80)NaX

0 200 400 600 8000

10

20

30

40

50

60

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(b)

Ag(90)NaX

0 200 400 600 8000

10

20

30

40

50

60

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(c)

AgX

0 200 400 600 8000

10

20

30

40

50

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(d)

Ag(80)NaX

0 200 400 600 8000

10

20

30

40

50

60

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(f)

AgX

Ag(90)NaX

0 200 400 600 8000

10

20

30

40

50

60

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(e)


127

Table 4.7. Adsorption capacity for Ag+ ion exchanged zeolite-A and X at 303 K and

760 mmHg.



CO CH4 N2 CO CH4 N2

NaA 24.7 13.55 7.5 13.54 7.43 4.11

Ag(62)NaA 49.8 16 7.8 37.67 12.05 5.87

Ag(70)NaA 56.6 18.2 8.7 47.59 15.30 7.31

AgA 58.9 35.3 19 51.71 30.67 16.65

NaX 27.17 14.00 9.2 14.71 7.58 4.98

Ag(80)NaX 56.7 19 10.5 44.24 14.82 8.19

Ag(90)NaX 58.9 23.72 14.32 47.76 19.21 11.59

AgX 65 26.3 16.9 54.70 22.13 14.22

Table 4.8. Adsorption selectivity for Ag+ ion exchanged zeolite-A and X at 288 and

303 K and 760 mmHg.

Sample Selectivity at 760 mmHg

288 K 303 K

CO/CH4 CO/N2 CH4/N2 CO/CH4 CO/N2 CH4/N2

NaA 2.06 3.32 1.62 1.82 3.29 1.81

Ag(62)NaA 2.60 5.01 1.93 3.11 6.38 2.05

Ag(70)NaA 2.44 5 2.05 3.11 6.51 2.09

AgA 1.60 2.59 1.62 1.67 3.1 1.86

NaX 1.62 2.88 1.78 1.94 2.95 1.52

Ag(80)NaX 2.69 4.35 1.62 2.98 5.40 1.81

Ag(90)NaX 2.28 4.05 1.77 2.48 4.11 1.66

AgX 2.11 3.53 1.67 2.47 3.85 1.56


128

4.3.6. Adsorption Isotherms and Selectivity for Ag+ Exchanged Zeolite-A

CO, CH4 and N2 adsorption isotherms on silver exchanged zeolite-A (AgA) at 288

and 303 K are given in Figure 4.3. Silver ion exchanged zeolite-A shows adsorption

behaviour similar to that of silver zeolite-X. AgA shows an equilibrium adsorption

capacity of 57.17, 35.5, 21.94 molecules/unit cell at 288 K for CO, CH4 and N2

respectively and 51.71, 30.67, 16.65 molecules/unit cell at 303 K and 760 mmHg for

CO, CH4 and N2 respectively. It is observed that on silver ion exchange zeolite A ,

CO, CH4 and N2 adsorption show more than threefold and fourfold increase on all

equilibrium pressures at 288 and 303 K respectively as compared to NaA. However,

the increase in the CO adsorption is very high at very low pressure due to

chemisorption. The CH4 adsorption capacity is higher than that of N2 at all

equilibrium pressures in the pressure range studied. In the low pressure region, the

CH4 and N2 adsorption capacity increases sharply with increase in pressure and the

adsorption isotherm posses a high slope as compared to NaA due to strong interaction

between gas molecules and silver clusters formed during vacuum activation after

more than 70% silver ion exchange. At equilibrium pressures above 380 mmHg, the

slope of the adsorption isotherms decreases.

CO, CH4 and N2 adsorption isotherms on different percentage of silver exchanged

zeolite-A at 288 and 303 K are given in Figure 4.3. The number within brackets

shows the percentage of silver ion exchanged in the zeolite-A, i.e. 62% and 70%,

silver exchanged zeolite-A are shown as Ag(62)NaA and Ag(70)NaA respectively. In

the case of zeolites having ≤70% silver exchange, i.e., for Ag(70)NaA and

Ag(62)NaA the CH4 and N2 adsorption isotherms obtained are similar to that of

NaA. For these samples the adsorption capacities for CH4 and N2 increases with

increase in the amount of silver. However the increase in adsorption capacities is

linear as silver ions do not interact as strongly as silver cluster and hence the shape of

adsorption isotherms remains linear.

The equilibrium adsorption capacities for the adsorption of CO, CH4 and N2 on

zeolite-A containing different amounts of silver ions are determined from the

adsorption isotherms and the values at 760 mmHg are given in Table 4.6 and 4.7. The

CO, CH4 and N2 adsorption capacity increases on silver ion exchange. However, after

exchanging more than 70% of the extra framework cations of zeolite-A with silver

ions, the CH4 and N2 adsorption capacity increase is sharp.


129

Figure 4.3 (a-f). Adsorption isotherms of CO, CH4, and N2 in silver ion exchanged zeolite-

A (a-c) at 288 K and (d-f) at 303 K.

The magnitude of the increase in the adsorption capacity for CH4 and N2 is much

higher than that of CO. The observed increase in the adsorption capacity follows the

order CH4 ≈ N2 >> CO. The Ag+ zeolite-A shows CO selectivity over CH4 and N2

which is very high in low pressure region and follows the order Ag(70)NaA ≈

0 200 400 600 8000

10

20

30

40

50

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(a)

Ag(62)NaA

0 200 400 600 8000

10

20

30

40

50

60

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(b)

Ag(70)NaA

0 200 400 600 8000

10

20

30

40

50

60

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(c)

AgA

0 200 400 600 8000

10

20

30

40

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(d)

Ag(62)NaA

0 200 400 600 8000

10

20

30

40

50

60

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(f)

AgA

0 200 400 600 8000

10

20

30

40

50

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(e)

Ag(70)NaA


130

Ag(62)NaA > NaA > AgA (Table 4.8). The decrease in CO selectivity for AgA was

due to the increase in N2 and CH4 adsorption capacity. The pure component

adsorption selectivity for CO over CH4 and N2, and CH4 over N2 at 760 mmHg

equilibrium pressures was calculated, and the values at 288 and 303 K are given in

Table 4.8.

4.3.7. Heat of Adsorption for Ag+ Exchanged Zeolite-A and X

The isosteric heat of adsorption for CO, CH4 and N2 were calculated from the

adsorption data at 288 K and 303 K and are given in Table 4.9. Heat of adsorption

calculated for the parent NaA and NaX are in close agreement with those reported in

the literature.13,14

All silver ion exchanged zeolite-A and X exhibit high heat of

adsorption for CO due to very strong interaction of CO molecules with silver cations.

Zeolite-A shows sudden increase in heat of adsorption for CH4 and N2 after 70%

silver ion exchange however the same was observed in zeolite-X after 80% silver ion

exchange. The heat of adsorption for CH4 and N2 on zeolite-A and X having less than

70% silver exchange was almost equal to that of NaA and NaX. The CH4 and N2 heat

of adsorption for AgA and AgX, decreases with increase in adsorption coverage due

to presence of different adsorption sites with different adsorption affinity. The heat of

adsorption for CH4 and N2, at low coverage is very high for AgA and AgX as

compared to that of the corresponding alkali metal ion exchanged zeolites. The heat

of adsorption for silver exchanged zeolites is even higher than those for bivalent

alkaline earth metal exchanged zeolites.

The sharp increase in the adsorption capacity in the low-pressure region (<250

mmHg), and high heat of adsorption for CH4 and N2 at low adsorption coverage is due

to the presence of co-ordinately unsaturated sites in the zeolite cavities, that interacts

strongly with CH4 and N2 molecules in silver exchanged zeolites. Further, CH4 and

N2 heat of adsorption shows a sharp decrease with increase in the adsorption coverage

reflecting the limited number of such sites.

The various interactions such as dispersion, polarization, field-dipole interactions,

field-quadrupole, close range repulsion interactions, and sorbate-sorbate interactions

are contributing towards the total energy of physical adsorption. The electrostatic

interactions between the sorbate molecules and extra framework cations of the zeolite

depend on the dipole moment, quadrupole moments and polarizability of the sorbate


131

molecule and are expected to follow the order, CO >> N2 ≈ CH4 in agreement with

dipole moment, quadrupole moment and polarizability (Table 1.1) of CO, N2, and

CH4. N2 has quadrupole moment less than CO; CH4 has no quadrupole moment but

has polarizability higher than CO and N2. Alkali and alkaline earth metal ion

exchanged zeolites A and X display the similar trend as observed from heat of

adsorption data for CO, N2, and CH4 in these zeolites. In fact, heat of adsorption at

zero coverage for CO, CH4 and N2 has been reported to show correlation with the

charge density of the extra framework cations.15-17

.CO, N2, and CH4 interact strongly

with the Li+ ions in the zeolite due to its higher charge density. Despite the same

charge and larger size (1.26Å) or lower charge density of the silver ions compared to

sodium cations (0.97Å), silver exchanged zeolites show stronger interactions with the

above gas molecules as observed from sorption capacity, selectivity and heats of

sorption data given in Tables 4.6-4.9.

Table 4.9. Heat of adsorption for Ag+ ion exchanged zeolite-A and X at 1

molecules/unit cell.

Sample Heat of Adsorption (kJ/mol)

CO CH4 N2

NaA 27 22 20

Ag(62)NaA 74 21 22

Ag(70)NaA 80 22 23

AgA 98 35 39

NaX 26 22 21

Ag(80)NaX 84 20 22

Ag(90)NaX 89 31 33

AgX 105 33 35

As discussed in the earlier section, silver exchanged zeolites-A and X on vacuum

dehydration at higher temperature form clusters that possess charge higher than +1.

The electrostatic interactions of these clusters with adsorbed CO, N2 and CH4

molecules will be higher than that with isolated Ag+ ions, which might be responsible

for higher heat of adsorption for these gas molecules in silver exchanged zeolites.

Further, it is reported17-20

that under prolonged evacuation at higher temperatures


132

(>623K) some of the Ag+ ions present inside the supercage undergoes reversible

intra-zeolite auto reduction to Ag0 by extracting charge from lattice oxygen, with

desorption of oxygen as reported from thermal studies. Ag0 migrates inside β-cage

and hexagonal prism, and interacts with Ag+ ions present there to form silver clusters.

If this occurs during the activation process, positively charged structural defects will

generated inside the zeolite cavities and will have electrostatic interaction with gas

molecules. The silver clusters in zeolites-A and X has been reported to be present in

the sodalite cage and, thereby, ruling out the possibility of direct interaction of the gas

molecules with silver clusters as CO, CH4 and N2 molecules due to their higher

kinetic diameter (Table 1.1) cannot enter the sodalite cages which have smaller pore

openings (2.2 Å). However two silver cations of the clusters are present in the six ring

of sodalite cage with Ag0 at the center. These cations are accessible to gas molecules

as single six member ring is also a part of super cage of zeolite-A. In case of AgX the

silver clusters are preferably formed in hexagonal prism (D6R) and later in single six

member ring (S6R). The silver cluster formed at D6R are directly not accessible for

adsorption while, silver cluster formed at S6R is accessible for gas adsorption and

responsible for high heat of adsorption as S6R is also a part of super cage of zeolite-

X.

The higher heat of adsorption for CO and N2 observed in AgA and AgX can also be

explained in terms of π-complexation of CO and N2 molecules with silver ions

present inside the zeolite super cage (Figure 4.4). From the electronic configuration of

N2 [KK (σ2s)2

< (σ2s*)2

< (σ2px)2 = (π2py)

2 < (π2pz)

2 < (π2px*)

0 = (π2py* )

0] and Ag

+ [Kr]

4d10

5s0, show the highest occupied and lowest unoccupied molecular orbitals in N2

molecule are the bonding (π2py), (π2pz) orbitals and antibonding (π2py*), (π2pz*) orbitals

respectively. Ag+ ions has completely occupied highest energy 4d orbitals along with

unoccupied 5s orbitals. The energy difference between the lowest unoccupied

molecular orbital of Ag+ ions present in zeolite and highest occupied molecular

orbital of N2 molecule is reported to be around 8eV. This facilitates electron transfer

by both σ-donation (electron transfer from bonding π2p orbitals of N2 molecules to 5s0

orbital of Ag+ ions) and d-π2p* back donation (electron transfer from completely

occupied 4d orbital of Ag+ ions to unoccupied π2p* of N2 molecule). This π-

complexation of N2 molecules with silver ions of the zeolites results into stronger

interaction between silver exchanged zeolite and N2 molecules.


133

The electronic configuration of C and O atom in CO is [KK (sp)C2, (sp)C

1, 2px

0 =

2py1], and [KK (sp)O

2, (sp)O

1, 2px2

= 2py1] respectively. The molecular orbital

electronic configuration for CO is [KK (sp)O2 , (σsp

b)

2, (πx

b)2 = (πy

b)2 , (sp)C

2 (πx*)

0 =

(πy*)0 , (σspz*)

0 ]. The C and O atom in CO molecule are sp-hybridized. Molecular

orbitals (sp)O2

and (sp)C2

are σ-non bonding molecular orbitals, (sp)O

2 present as lone

pair of electron on ‘O’ atom of CO molecule has low energy, more s-character and is

very stable ( i.e. unreactive) hybrid orbital. However (sp)C2 present as lone pair of

electron on ‘C’ atom of CO molecule has higher energy, more p-character and is very

unstable ( i.e. reactive) hybrid orbital. Because of the presence of this highly reactive

non bonding (sp)C2 electrons CO molecule act as a strong ligand and can coordinate

very easily and strongly with silver metal and ions. The highest occupied and lowest

unoccupied molecular orbitals in CO molecule are the non bonding (sp)C2 and

antibonding (π2px*), (π2py*) orbitals respectively.

Figure 4.4. Metal ligand (M-CO) bonding during CO adsorption in transition metal ion

exchanged zeolite-A and X.

The high adsorption capacity and heat of adsorption for N2 was observed at higher

silver exchange, as the silver ions either migrated in the sodalite cages or reduced to

metallic (Ag0) form during activation and are not accessible for π-complexation in

low silver exchange zeolite-A and X and the silver clusters which forms strong π-

complexation are also formed at higher silver exchange. However, CO molecule show

very high adsorption capacity and heat of adsorption even at low silver exchange due

to "synergic π* back-bonding" that requires metal with d-electrons, and in a relatively

low oxidation state (Ag0, ag

+). Moreover CO also forms clusters by bonding with

more than one silver atom or ions and hence show high adsorption capacity and heat

of adsorption. The π-bonding has the effect of weakening the carbon-oxygen bond

compared with free carbon monoxide. Because of the multiple bond character of the

M-CO linkage, the distance between the metal and carbon is relatively short, often <


134

1.8 Å, about 0.2 Å shorter than a metal-alkyl bond. The C-O vibration, typically

called νCO, occurs at 2143 cm-1

for CO gas. The positions of the νCO band(s) for the

metal carbonyls are inversely correlated with the strength of the pi-bonding between

the metal and the carbon and hence the vibrations occur below 2143 cm-1

.

The variations observed for CO, CH4, and N2 adsorption in terms of the adsorption

capacity and heats of adsorption for different amount of silver exchanged zeolite A

(Tables 4.6-4.9) can be explained in terms of difference in number of accessible/co-

ordinately unsaturated Ag+ cations present in the zeolite. The adsorption capacity is

much higher (Table 4.9) for fully exchanged AgA and AgX as compared to NaA and

NaX in the pressure range studied.

The heat of adsorption for N2 and CH4 on percentage silver exchange in zeolite-A

(Table 4.9) shows an exponential rise at around 70% silver exchange. These sharp

increase in heat value show that more active sites in zeolite A arises only after 70%

sodium cations are exchanged with silver cations. It is reported that, in Ag12A, three

Ag+ ions present within the sodalite unit of the hydrated form, move closer to the

planes of the nearest 6-rings upon dehydration.15-19

Simultaneously, the Ag+ ions

present at the 4-ring site, and at the 8-ring sites undergo reduction and these Ag+ ions

become nearly zero coordinate, 2.9 Å from the nearest framework oxide ions. The

sum of the ionic radii of Ag+ and O

2- is 2.6 Å and the other Ag-O bond distance in the

zeolite structure range from 2.2 to 2.5 Å. Therefore, the Ag+ ions present at the 4-ring

site, and at the 8-ring sites with Ag-O bond distance of 2.9 Å are least adequately

coordinated Ag+ ions and are the energetically potential adsorption sites in the

supercage. In terms of the accessibility of Ag+ ions, cations located at 6-ring, 4-ring

and 8-ring are expected to interact with CO, CH4 and N2 molecules as normally

observed for sodium or calcium cations present at these locations. However, the

factor, which makes CO, CH4 and N2 molecules interaction with silver cations

stronger, is the presence of coordinately unsaturated Ag+ ions at 4-ring and 8-ring

locations, which can have stronger π-complexation with CO and N2 molecules as

explained in earlier section.

Nearly similar heat of adsorption observed for NaA and AgNaA samples (Table 4.9)

having less than 70% silver exchange can be explained in terms of locations of Ag+ in

partially silver exchanged zeolite-A. It has been reported that in partially silver


135

exchanged zeolite Na4.4Ag7.6A, dehydrated under vacuum at 643 K, three sodium ions

occupy 8-ring sites and the remaining 1.4 sodium and 6.6 silver ions located at 6-ring

sites and one reduced silver per unit cell is located in sodalite cage.15

Thus, no Ag+

ions are present at 4-ring and 8-ring sites, which due to their unsaturated coordination

react strongly with CH4 and N2 molecules.

Framework structure of zeolite-X along with the extra framework cation locations is

given in Figure 2.1. The cations are located in six crystallographically different sites.

The cation locations of Ag+ ions in zeolite AgX has been reported by Lee et al

16 from

X-ray diffraction studies. In AgX, 32 cations are located either at site I (centre of the

hexagonal prism connecting the sodalite ages) or site I′ (near the 6-ring window of the

prism on the inside of sodalite cage). The other 32 Ag+ ions are located at site II (on

the either side of the single 6-ring window between the sodalite and supercage). The

distance between these Ag+ cations and framework oxygen is reported to be 2.273 Å,

similar to that between Ag+ cation at 6-ring and framework oxygen in Ag12A.

13-15 23

Ag+ ions are located at three different III′ sites (opposite 4-ring near the wall of the

supercage or the edge of 12-ring). These Ag+ ions are located at three different III′

sites with Ag-O distances 2.702, 2.31, and 2.45 Å.

Cations present at site I or I′ are inside the hexagonal prisms and are not accessible to

N2 molecules. So they do not contribute towards N2 adsorption. Of the accessible

cations at site II and III′, the cations located at site III′ with a Ag-O distance 2.702 Å

will be co-ordinately unsaturated and cations located at other sites (II or remaining

III′) are strongly interacting with framework oxygen of the zeolite as observed from

the Ag-O distances 2.27-2.45Å. Therefore, in AgX, the Ag+ cations located at site III′

with Ag-O distance 2.702 Å being co-ordinately unsaturated will strongly interact

with N2 molecules through π-complexation. Hutson et al.17-19

also explained the

stronger N2 adsorption in AgX in terms of more accessible Ag+ cations located at site

II*, as termed by them, from Neutron diffractions studies. For partially or less than

80% silver exchanged zeolite-X no Ag+ ions are present at co-ordinately unsaturated

S III’ sites and hence they show low heat of adsorption and low adsorption capacity.

4.3.8. Dynamic Adsorption Studies of CO, CH4, N2 from Binary Gas Mixtures

The pore openings and cage structure of sodium and silver ion exchanged zeolite-A

and X are large enough to neglect any steric effects of the adsorbate with the


136

0 10 20 30 40 50 60 70 80 90 100

0.0

0.3

0.6

0.9

1.2

C/C

0

Time (min)

CH4 Adsorption

CH4 Desorption

(d)

0 5 10 15 20 25

0.0

0.3

0.6

0.9

1.2

C/C

0

Time (min)

CH4 Adsorption

CH4 Desorption

(a)

0 5 10 15 20 25 30 35 40 45

0.0

0.3

0.6

0.9

1.2

C/C

0

Time (min)

CO Adsorption

CO Desorption

(b)

0 5 10 15 20 25 30 35 40 45

0.0

0.3

0.6

0.9

1.2

C/C

0

Time (min)

CO Adsorption

CO Desorption

(c)

0 20 40 60 80 100 120 140 160 180

0.0

0.3

0.6

0.9

1.2

C/C

0

Time (min)

CO Adsorption

CO Desorption

(e)

0 20 40 60 80 100 120 140 160 180

0.0

0.3

0.6

0.9

1.2

C/C

0

Time (min)

CO Adsorption

CO Desorption

(f)

Figure 4.5 (a-f). Dynamic breakthrough curves for AgX: (a) CH4+N2 (b) CO+N2 and (c)

CO+CH4 and AgA: (d) CH4+N2 (e) CO+N2 and (f) CO+CH4 gas mixtures.

adsorbent structure at 303 K. However the cationic position and nature in the zeolite

is the most important cause of the difference in dynamic adsorption capacity of CO,

CH4 and N2. The breakthrough measurements for CO, CH4 and N2 from binary gas


137

mixture have been carried out on AgA, and AgX at 303 K, 1 atm pressure and feed

flow of 100 ml/min. The breakthrough data are given in Table 4.10. The CH4 + N2,

CO + N2 and CO + CH4 binary adsorption and desorption breakthrough curves for

AgA and AgX are shown in Figure 4.5. The binary adsorption study shows the CH4

and CO selectivity over N2, and CO selectivity over CH4, for both silver exchange

zeolite-A and X in accordance with equilibrium selectivity. For CH4 and N2 gas

mixture separation silver zeolite-X is a good adsorbent as it shows higher

breakthrough capacity with easy desorption, while silver zeolite-A shows best result

for CO separation from CH4 and N2.

Table 4.10. Breakthrough data for binary gas mixtures on AgA and AgX.

Adsorbent Feed gas

composition

by volume (± 1%)

Weight of

adsorbent (g)

Break-

through

Time (min)

Dynamic

capacity (cm

3/g)

Maximum

temp.

during

adsorption (K)

Minimum

temp.

during

desorption (K)

AgA CH4 = 68 N2 = 32

121.5 10 5.6 305 302

AgA CO = 78 N2 = 22

121.5 60 38.5 333 299

AgA CO = 63 CH4 = 37

121.5 65 35.3 328 299

AgX CH4 = 68

N2 = 32 63.1 8 8.6 306 301

AgX CO = 78 N2 = 22

53.2 24 35.3 330 299

AgX CO = 63 CH4 = 37

53.2 22 26.2 324 300

4.3.9. Dynamic Adsorption Studies of CO, CH4, N2 from Ternary Gas Mixtures

The breakthrough measurements for CO+CH4+N2 ternary gas mixture was carried out

on AgA and AgX at 303 K and 1 atm pressure and feed flow of 100ml/min. The

breakthrough data are given in Table 4.11. The ternary adsorption and desorption

breakthrough curves for AgA and AgX are shown in Figure 4.6. The dynamic

adsorption studies showed that both AgA and AgX have CO selectivity over CH4 and

N2 with breakthrough capacity of 34.9 and 31.4 cm3/g, respectively, and CH4

selectivity over N2.


138

The increased concentration of CH4 in outlet during adsorption was due to

replacement of initially adsorbed CH4 with high affinity CO molecules. On Ag+ ion

exchange the CO dynamic adsorption capacity increases due to increase in the CO

adsorption capacity of silver exchanged zeolites. The breakthrough adsorption

capacity is less than equilibrium adsorption capacity due to decrease in partial

pressure, competitive adsorption between gas molecules and decrease in adsorption

time. Due to strong adsorption of CO with silver zeolite the adsorbed gas was not

easily desorbed by counter current purging of N2 at 100 ml/min and takes very long

time for desorption.

Figure 4.6 (a-d). Dynamic breakthrough curves for ternary (CO+CH4+N2) gas mixture on

(a) AgX adsorption, (b) AgX desorption, (c) AgA adsorption and (d) AgA desorption.

0 30 60 90 120 150 180

0

20

40

60

80

100

120

Vo

lum

e (

%)

Time (min)

CO

CH4

N2

(a)

Zeolite AgX (Adsorption)

0 30 60 90 120 150 180

0

20

40

60

80

100

120

Vo

lum

e (

%)

Time (min)

CO

CH4

N2

(b)

Zeolite AgX (Desorption)

0 30 60 90 120 150 180

0

20

40

60

80

100

120

Vo

lum

e (

%)

Time (min)

CO

CH4

N2

(c)

Zeolite AgA (adsorption)

0 30 60 90 120 150 180

0

20

40

60

80

100

120

Vo

lum

e (

%)

Time (min)

CO

CH4

N2

(d)

Zeolite AgA (Desorption)


139

Table 4.11. Breakthrough data for ternary gas mixtures on AgA and AgX.

Adsorbent Feed Gas

composition

by volume

(± 1%)

Weight of

adsorbent

(g)

Break-

through

Time

(min)

Dynamic

capacity

(cm3/g)

Maximum

temp.

during

adsorption

(K)

Minimum

temp.

during

desorption

(K)

NaA CO = 53 CH4 = 32 N2 = 15

97.3 19 10.4 306 302

AgA CO = 53 CH4 = 32

N2 = 15

121.5 80 34.9 325 299

NaX CO = 53 CH4 = 32 N2 = 15

86.5 9 5.52 305 302

AgX CO = 53 CH4 = 32 N2 = 15

111.3 66 31.4 326 299

4.4. Conclusions

Equilibrium adsorption measurements of CO, CH4 and N2 are performed in zeolite-X

and A exchanged with different transition metal ions. The adsorption capacity and

crystallinity decreases on Mn2+

, Co2+

, Ni2+

, Cu2+

, Zn2+

, Ru3+

, Rh3+

, Pd2+

, and Cd2+

exchange in zeolite-X. However silver exchanged zeolite-A and X shows good

adsorption capacity with retaining the crystallinity. In silver zeolite-A and X the

adsorption of CO is higher than that of CH4 and N2 molecule. The adsorption

isotherm clearly shows CO selectivity over CH4 and N2 which is very high in low

pressure region, and CH4 selectivity over N2. High heat of adsorption for CO was

observed for silver ion exchanged zeolites. Sharp increase in CH4 and N2 heat of

adsorption was observed in silver zeolite-A and X after more than 70% and 80%

exchange respectively due to formation of silver clusters at high temperature vacuum

activation and -complexation. Due to higher quadrupole moment, polar nature and

-complexation, and carbonyl formation, adsorption capacity and heat of adsorption

of CO is very high even at low silver exchange.


140

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